The present invention relates to temperature sensitive electrical devices and, more particularly, to a temperature sensitive switch device which is normally open but which closes an electrical circuit when the device is raised to a predetermined temperature.
Thermally actuatable switch constructions are presently known in which an electrical circuit is broken or closed in response to the switch being heated to a predetermined temperature. In U.S. Pat. No. 3,875,546, issued Apr. 1, 1975, to Merrill, and U.S. Pat. No. 3,519,972, issued July 7, 1970, to Merrill, both assigned to the assignee of the present invention, temperature responsive electrical switches are disclosed in which a sliding electrical contact is held against a second electrical contact by a relatively stiff spring, which spring bears upon a normally solid, heat fusible pellet. When the switch is raised to the required temperature and the pellet melted, the stiff spring no longer opposes the force of a somewhat weaker spring which then moves the sliding contact away from the second contact, thus opening an electrical circuit.
A similar switch is shown in U.S. Pat. No. 3,180,958, issued Apr. 27, 1965, to Merrill, and assigned to the assignee of the present invention. One embodiment disclosed in the Merrill U.S. Pat. No. 3,180,958 patent includes a member of heat fusible material which holds a sliding contact away from a second contact until the fusion temperature is reached. While sliding contact, thermal switching devices of this type are extremely reliable in operation, there is always a possibility that the moving parts may bind. Additionally, thermal switches of this type require a significant number of small parts to be assembled and, therefore, the assembly costs are not insignificant.
Heat sensitive switching devices of the type used as a fuse in many electrical circuits include a heat fusible electrical conductor which conducts current between two electrodes. When the conductor is raised to its fusion temperature as a result of carrying an excessive current load, it melts and breaks the electrical circuit between the electrodes. Other fuse-like mechanisms are also known in which heat for melting an electrical conductor is supplied externally rather than by resistance heating of the conductor itself. A simple thermal switching device for closing an electrical circuit when heated to a predetermined temperature, regardless of the orientation of the switch, has not previously been available.
Thus a need exists for a temperature sensitive switching device which is simple in construction, reliable in operation, and is capable of closing an electrical circuit when raised to a predetermined temperature, regardless of the orientation of the device.